Yo. Unicorn Smasher founder here. Our product is fairly accurate, but since our recent release was version #1, there will be room for improvement.
I've personally used Jungle Scout for a long time, and I'm a big fan of it. I'm certain that their data is a little more accurate right now since they've been able to tweak it for a long time now. Unicorn Smasher is more like the 80/20 of what you need right now, but I would never deter anyone from using JS. Solid product. I would recommend just using both if you already have a paid version of JS, as there will be spots where each tool is superior.
1) Yeah the monthly storage fee didn't seem right. If your product doesn't sell, you incur long term storage fees after 6 months. If your item is small you can store some in your house and just ship whenever you feel like it. Amazon's UPS rates to ship to warehouse are really cheap
2) Yes even prime members have add-on items. Some items are cheap and aren't add-on, but usually they get flagged as add-on later. Anything $9.99 and under is susceptible to having add-on status
3) Good
4) You can use JungleScout ($$$) or you can use Unicorn Smasher which is free. It's a Chrome extension http://www.unicornsmasher.com/
5) Well there's no benefit to selling generic unless you're hijacking someone else's generic listing. If you're generic, anyone can sell on your listing and take the buy box from you and edit your listing and change the pictures, etc. Which is why most PL sellers brand their items and get brand registry even if you're just putting a sticker on your packaging